I've used the old, broken up concrete as a "center" for new step placements. Supplemented by any large boulders in the neighborhood. The trick is not to go overboard with any fill you put in there, or the quality of the step may suffer if the finished concrete is too thin (especially if you're too stingy with your rebar).

Do not use loose dirt because it will settle. Cinder blocks (chunks and pieces?) will work fine. Clean rocks are great. Tamp the cement deeply to settle the pour. I like vibrating my foundation cement if you can access a concrete vibrator.

Make sure your make a continuous pour so everything bonds. Don't pour one day and then pour hours later or even on another day.